1. Field of the Invention
The present invention relates generally to a magnetic reluctance flowmeter for measuring the rate of flow of gasses or liquids, and relates more particularly to a magnetic reluctance flowmeter having a magnetic float within the flow stream and a yoke outside the flow stream, wherein the yoke is positioned relative to an electronic circuit having magnetic field sensors arranged to produce a signal which is related directly to the displacement of the magnetic float caused by the flow of the stream.
2. Description of the Related Art
The rate of flow of gasses or liquids flowing in a pipe can be measured by installing a partial obstruction in the pipe to create a differential pressure. Pressure is directly related to the square of the flow velocity according to the well known Bernoulli's Equation, and thus the difference in pressure before and at the obstruction can be used as a measure of the flow velocity. Because the differential pressure depends upon the geometry of the obstruction, the range of flow velocity that can be measured with such a device is limited, and this inhibits its ability to measure low flow velocities to within a reasonable accuracy of 1% or less. Measured pressures at the installed partial obstruction may be also adversely affected by particles within fluids and/or diameter changes or angled portions of installed pipe sections. Additionally, since it is desirable to provide a signal which is linearly related to the flow velocity, and because this method is based upon the non-linear relationship between flow and pressure, a secondary measuring system is required to convert pressure into the desired linear signal.
A direct linear measurement of the rate of flow of gasses or liquids flowing in a pipe is achieved by installing a turbine rotor in the pipe. The speed of rotation of the rotor is measured by an appropriate sensor mounted adjacent to the rotor and outside the pipe, such that each blade of the rotor induces a signal pulse in the sensor and the rate of these pulses is interpreted to give a linear measurement of the flow over moderate to high velocities. The range of flow velocity that can be measured with such a linear device is around 10:1. A limitation of this method is that the turbine rotor is mounted in bearings, the friction in which causes the relationship between flow velocity and rotor speed to become non-linear at low flow velocities.
A linear measurement of the rate of flow of gasses or liquids flowing in a pipe is provided by a float which travels up and down a short vertical section of pipe having a tapered bore of variable area. In the art, such apparatus are commonly referred to as rotometers. The pressure on the float caused by a progressively increasing flow passing upwards in the pipe causes the float to rise progressively into pipe sections of greater cross-sectional area, thence to settle in a position where this pressure is in equilibrium with the gravitational force acting on the float. If the section of pipe is transparent and the fluid is not opaque, the height to which the float rises is proportionally related to the flow velocity, but this is a visual measurement which can only be translated into a signal with extreme difficulty. However, the range of flow velocity that can be measured with such a linear device is again around 10:1, and this facilitates the measurement of low flow velocities to within a reasonable accuracy of 1% or less.
While there are many other complex devices for measurement of the rate of flow of gasses and liquids in pipes, an advantage of the aforementioned float method is its simple construction with a single moving component (the float) in the flow stream. Attempts to alleviate the one significant disadvantage of the float method, that it is essentially a visual measurement method, have resulted in the use of permanent magnets as the float. German Laid-Open Patent Numbers DE 3,411,156 and DE 3,505,706 disclose a magnetic field resulting from a permanent magnet float will exist outside the pipe, where it may interact with a magnetic field sensor (or sensors) which operates in conjunction with an electronic circuit. Clearly the movement of the permanent magnet float within the flow stream must be constrained within a limited linear distance, and as the float is displaced by the flow, the resulting magnetic field will modify the signal produced by the electronic circuit. Thus, an electrical signal is produced which is related to the flow velocity. However, by not constraining the resulting magnetic field in a well defined magnetic circuit, the magnitude of the field at the sensor for any fixed position and orientation of the sensor will vary in a highly non-linear manner with the flow velocity, and it may be influenced by any other magnetic components surrounding the device. Consequently, by using a permanent magnet float as described in the prior art, the advantage of producing an electrical signal as a function of flow velocity is accompanied by the disadvantage that this function is now non-linear and somewhat unpredictable.